Gray squares in the corners of my LibreOffice document

I’m seeing gray squares in the corners of my LibreOffice document. I’m not sure what they are or how to remove them. Does anyone know what causes this and how to fix it?

Version: 25.8.2.2 (X86_64) / LibreOffice Community
Build ID: d401f2107ccab8f924a8e2df40f573aab7605b6f
CPU threads: 8; OS: Linux 6.14; UI render: default; VCL: gtk3
Locale: nl-NL (nl_NL.UTF-8); UI: nl-NL
Flatpak
Calc: threaded

Edit: Here is a ODF sample file:
TestFile.odt (7.6 KB)

Please upload a real ODF type sample file here.

tdf#154443